1. Field of Invention
The invention pertains to the field of pressure sensing, and more particularly to determining the level of a liquid above a reference plane and the weight of an object with the use of pressure sensors.
2. Description of the Prior Art
Various sensor types have been employed in the prior art for sensing the level of a liquid in both stationary and mobile tanks. These prior art sensors have utilized mechanical, electromechanical, electromagnetic, electronic, and sonic systems to determine liquid levels. Problems exist in each of these systems. Mechanical systems using floats and levers are not sufficiently accurate for most applications. These systems have additional problems in large tanks where guide wires or large strip arrays must be used to cover large areas. In such systems the floats and levers can xe2x80x9chang upxe2x80x9d, further degrading the system accuracy.
Electronic liquid level measuring systems having electronic strip sensors are hazardous in highly combustible liquids in which a slight spark can cause a major conflagration. Measuring the level of caustic liquids presents a problem for both mechanical and electronic systems. The caustic liquid can xe2x80x9ceat awayxe2x80x9d at the floats, levers, and electronic strips adversely affecting the system calibration, thereby further degrading the system accuracy.
Liquid level measuring devices using sonic or ultrasonic signals are complex when used to measure levels of contained liquids because of echos received from the walls of the container and layers of impurities in the liquid. These deficiencies are overcome by electromagnetic systems which utilize subnanosecond pulses and coaxial transmission lines, thus eliminating wall echos. One such system uses a slotted coaxial line. The slot, however, tends to clog and requires frequent cleaning. This problem is over come by eliminating the slot with the use of a single wire transmission line. Since these systems determine the level of the liquid by detecting reflections of electromagnetic waves from the liquid surface, they do not actually measure the liquid level above the floor of the tank.
An electromagnetic device that overcomes the deficiencies of the coaxial and single wire transmission systems uses a combination of single wire and coaxial transmission lines to detect reflections from the floor of the tank and the surface of the liquid. The difference between the arrival times of the two reflected signals is a measure of the liquid level above the floor of the tank. This system, however, is extremely complex and expensive.
All of the above deficiencies are overcome by the invention disclosed in U.S. Pat. No. 5,026,984 issued to David W. Gerdt on Jun. 25, 1991 and assigned to the assignee of the present invention. This invention uses a variable ratio pressure sensitive fiber optic coupler which is useful in a wide range of tanks and liquids. Gerdt""s liquid level sensor, however, is extremely dependent on the characteristics of the specific sensor employed. Further, the invention disclosed by Gerdt does not account for pressure exerted on the surface of the liquid. Thus, it is not self calibrating and has an inherent error in the determination of the liquid level.
A liquid level and weight sensor in accordance with the present invention utilizes a pressure sensor of any type positioned at the base of the liquid or weight. A second pressure sensor is positioned external to the liquid or weight to measure the pressure exerted on the liquid or weight. This external pressure is subtracted from the total pressure indicated by the base positioned pressure sensor to determine the actual pressure exerted by the liquid or weight. The actual pressure is then utilized in the calculation of liquid level or weight.
The determination of liquid level requires that both the pressure and density of the liquid be known. When the density of the liquid is not known, a third pressure sensor is inserted in the tank at a known level above the first.
The differential pressure and differential level are determinative of the liquid""s density.